Non-dispersive infrared radiation (NDIR) measurement techniques have been known for many years as highly useful in analysis of certain gas mixtures. More particularly, infrared radiation is absorbed selectively by certain gas species of interest in a wide variety of circumstances. For example, NDIR measurement of the absorptivity of infrared radiation by carbon dioxide-containing gas mixtures is employed by instruments for measuring the total organic carbon content of water. NDIR measurements of carbon dioxide and other gases, such as carbon monoxide, are also commonly used to monitor atmospheric conditions and to analyze automotive exhaust gases.
More particularly, the total organic carbon content of a sample of water can usefully be measured by adding oxidizers to the water, oxidizing the carbon in the water by exposure to ultraviolet radiation, and separating the carbon dioxide thus formed from the water sample by diffusion across a gas-permeable, water-impermeable membrane into a stream of carrier gas of known composition, commonly carbon dioxide-free nitrogen. This mixed gas sample is then admitted to the cell of an NDIR instrument for measurement of the proportion of carbon dioxide present.
To measure the amount of, for example, carbon dioxide in a mixture with nitrogen employing the basic NDIR principle, an infrared source is provided at one extremity of a closed cell containing the mixture and a suitable detector at an opposite extremity of the cell. Because carbon dioxide absorbs infrared radiation of certain wavelengths, and nitrogen does not, the concentration of carbon dioxide present in the mixed gas sample can be selectively measured by measurement of the transmission through the sample of infrared radiation at wavelengths absorbed by carbon dioxide.
Typical NDIR instruments measure carbon dioxide absorption in a narrow bandwidth around 4.3 microns. Wavelength selectivity is commonly achieved through the use of multiple-layer interference filters between the sample and the detector. If an on-line calibration process is carried out periodically, e.g., using pure nitrogen to "zero" the instrument, or if another method is provided to ensure that the intensity of the source remains constant over time (variations in the infrared source being the most significant cause of long-term drift) accurate relative measurements of the carbon dioxide content of the mixed gas stream can be made.
Infrared detectors responsive to 4.3 micron, i.e., 4300 nm, radiation include quantum photovoltaic and photoconductive detectors, gas-filled detectors such as bolometers and Golay cells, and thermally-responsive thermopiles and pyroelectric detectors. Of these, pyroelectric detectors are the most compact and least expensive, and are therefore commonly used in low-cost NDIRs.
Pyroelectric detectors comprise a piezoelectric crystal arranged to be heated by the incident radiation to be measured. The crystal provides a voltage signal responsive to the rate of change of the temperature of the crystal, e. g., due to heating by the incident radiation; the signal is independent of the wavelength of the incident radiation. As the output signal is responsive to the rate of change of the temperature of the crystal of the pyroelectric detector, a modulated light source is required to produce an output responsive to absorption of radiation by a gas sample. Modulation is typically accomplished either by employing a mechanical chopper, e.g. a spinning disk with slots disposed in the path of the light between the source and detector, or by electrically modulating the light source on and off.
The usual source of 4.3 micron light in NDIR instruments is an electrically heated element that serves as a black-body radiator. Silicon carbide rods have been used as low color temperature filaments, that is, to provide a 350-400 degree Kelvin, i.e., K. source whose maximum intensity is about 4.3 microns. Nichrome and Kanthal filaments have also been used, either in air or an inert atmosphere, as low color temperature infrared sources. These elements typically have relatively high thermal capacities and therefore require the use of an optical chopper to modulate the light.
Recent low-cost NDIR instruments have employed the low thermal capacity filaments of standard miniature incandescent lamps as high color temperature sources that can be electrically modulated, thereby eliminating the need for bulky and mechanically unreliable optical choppers. Although most of the radiation emitted by these lamps is in the visible portion of the spectrum, far outside the infrared range of interest in NDIR instruments, the low thermal mass of their filaments and the fact that they operate in a vacuum nonetheless leads to greater efficiency than low color temperature alternatives. Additional measures, however, are generally required to effectively block the visible portion of the spectrum.
Interference filters for CO.sub.2 detection, that is, as used to ensure that only radiation absorbed by CO.sub.2 is incident on the detector, are also readily available, either integrated directly into a pyroelectric detector package or as a separate window. The -113 filter from Eltec Instruments, Inc., Daytona Beach, Fla., for example, is an optional filter for their pyroelectric detectors that is specifically offered for CO.sub.2 monitoring; this filter has a transmission bandwidth from 4.183 to 4.353 microns. However, as discussed in detail below, this filter is not optimal for this purpose.
NDIR instruments now available fall generally into several classes. A first class of relatively expensive NDIR instruments typically employs an infrared-emitting filament as the source, a mechanical chopper to modulate the beam, and one or two infrared detectors, with suitable filters.
Instruments comprising two detectors typically include two separate cells defining beam paths of equal length. One cell is filled with a non-infrared-absorptive gas, providing a "reference path", and the other with the sample; the signals provided by the detectors are then compared in a ratiometric determination of the absorption. Such instruments can be made to work well, but are complex, bulky, and expensive. Furthermore, accurate measurements can be made only if the temperatures of both detectors are maintained equal, and if the detectors age in substantially identical fashion over time; neither condition can be ensured conveniently. Moreover, as the signals provided by pyroelectric detectors are relatively noisy, such two-detector instruments inherently possess substantially more noise than single-cell instruments.
Comparatively less expensive single-cell NDIR instruments now available commonly employ an electrically-modulated incandescent lamp mounted at one end of a tube as the infrared source, and a pyroelectric detector comprising an interference filter at the opposite end to measure the amount of infrared radiation passing through a sample in the tube. The inner surface of the tube is commonly gold-plated to ensure consistent high reflection of the infrared radiation as it travels along the tube from the source toward the detector.
A first type of single-cell instruments, which are relatively inexpensive and therefore popular, do not provide any compensation for instrument drift over time. These instruments typically use an offset gain stage, wherein the signal responsive to infrared intensity is subtracted from a constant and then amplified. In this manner an attenuation of between zero and, for example, 10% of the incident light intensity can be mapped as a zero-to-full-scale output. As described in detail below, the output of this type of system is normally a difference voltage. The principal disadvantage of this method is that because the output difference voltage is proportional to the incident light intensity, as the lamp ages and its output changes, the readings will change in proportion producing substantial gain errors. A second disadvantage is that the output is proportional to the system gain, causing the output to drift with component aging or temperature-induced changes in the gain stages. A third disadvantage is that lamp aging and the corresponding reduction in light output substantially reduce the dynamic range of the instrument.
A more sophisticated form of single-cell instruments, on the other hand, typically provide movable filters or other means for controlling the wavelength of the radiation incident on the sample between a first "reference" wavelength not absorbed by the sample and a second "measurement" wavelength that is absorbed by the sample; a ratiometric calculation is then made to determine the proportion of absorptive gas in the sample. Such instruments involve moving parts and are subject to various mechanical difficulties, leakage, and the like. Further, although the noise in such systems is low relative to that exhibited by the dual-detector instruments discussed above, and use of a single detector eliminates problems inherent in use of two detectors that may or may not be precisely matched, no provision is usually made to ensure the long-term stability of the source.
Further improvements in NDIR instruments of this type are shown in FIG. 5 of Small et al, "Oxidation and detection techniques in TOC analysis", Am. Lab. 18(2), February 1986, pp. 141-150; Small suggests that automatic gain control can be used to control the lamp intensity, presumably to improve the long-term accuracy.
Other known NDIR instruments combine elements of both types discussed above. For example, Passaro et al U.S. Pat. No. 4,687,934 shows an NDIR instrument adapted for measurement of several components of automobile exhaust streams. (Earlier Passaro U.S. Pat. Nos. 4,346,296 and 4,398,091 are generally similar.) The Passaro instrument employs a mechanical chopper to modulate the infrared radiation from the source, while several detectors are provided at the opposite end of a sample tube. Each detector is provided with a different interference filter, rendering the detectors selective for the exhaust components of interest. Calibration is provided by introduction of either a "zero" gas, i.e., one known to be infrared-transparent, or a sample gas of known composition, and adjusting the instrument output accordingly, and is accomplished either at predetermined intervals or when a thermistor indicates a drift in ambient temperature of more than 6.degree. C. The degree of inaccuracy inherent in this approach would be unacceptable in many applications.
The measurements of concentration of carbon dioxide (and other gases) in gas samples provided by all prior art NDIR instruments known to the inventor also suffer from certain inherent inaccuracies in the signal processing techniques employed, as follows.
A first source of inaccuracy inherent in the design of all known prior art NDIR instruments occurs when these instruments employ a well-established equation known as Lambert's Law to calculate the CO.sub.2 concentration, as follows: EQU I=I.sub.0 e.sup.-abc
where:
I=measured light intensity at the detector PA1 I.sub.o =light intensity at the source PA1 a=absorption coefficient PA1 b=path length PA1 c=CO.sub.2 concentration PA1 where: PA1 where L=Light Leakage factor PA1 where:
In fact, the present inventor has determined that the direct application of Lambert's Law, as usually applied to measurements of carbon dioxide by measuring the absorption of infrared radiation from a non-monochromatic source, is not accurate. (The same is true of the usual use of Lambert's Law to measure the concentration of other gases.) The inaccuracy arises because the absorption by CO.sub.2 of infrared radiation in a band of wavelengths centered around 4.3 microns is incomplete. Rather than absorb a wide bandwidth of wavelengths centered on 4.3 microns, the absorption spectrum of CO.sub.2 is comb-like, as shown in FIG. 7 of this application. Therefore, a substantial fraction of the wide-bandwidth infrared radiation provided by the usual incandescent filaments or other black-body radiators "leaks" past the CO.sub.2 and heats the detector. As the usual calculations performed to determine the CO.sub.2 effectively assume that the absorbtion is a simple function of the CO.sub.2 concentration, the measurements are inaccurate.
Research by the present inventor has shown that this light "leakage", i.e., the proportion of the infrared radiation that is within the passband of commercially available interference filters designed specifically for CO.sub.2 absorption measurements, but is not absorbed by CO.sub.2, ranges from over 90% (worst case) to about 70% (best case) of the incident light. Hence, regardless of the CO.sub.2 concentration, the detector will register significant radiation in the band assumed to be absorbed by CO.sub.2. Unless the signal is compensated in some way, the detector's output will underreport the CO.sub.2 actually present in the sample; that is, the "leakage" will be misinterpreted as a less-than-correct value for the CO.sub.2 content. Most manufacturers make empirical corrections to linearize the readings provided by their instrument, i.e., correct the measurements procided by their instruments to correspond to to calibration measurements performed using samples of known CO.sub.2 content. However, to properly and accurately address this problem, a physical theory is needed that addresses the noted deviation.
Second, the interference filters used in commercially available NDIR instruments for measurement of CO.sub.2 generally do not take into account the effective frequency shift in the filter passband caused by the wide angle of incidence of radiation on the interference filter. A significant fraction of the infrared radiation transiting a gold-plated sample tube reflects repeatedly from the inner wall of the tube and therefore exhibits a wide angle of incidence on a filter at the end of the tube. This phenomenon has the effect of shifting the pass band of the filter away from the absorption band of CO.sub.2, thereby reducing the signal-to-noise ratio.
A further noise-related problem inherent in the design of presently-available NDIR instruments results from inappropriate selection of the lamp drive frequency as effectively required by limitations on the signal processing circuitry typically employed. More specifically, the frequency response of the typical pyroelectric detector has a flat peak extending from around 0.05 hz to about 1 hz, above which it drops off at 6 db/octave, while the emission efficiency of miniature lamps typically employed as infrared sources peaks at about 1 hz. Accordingly, it would be preferred to operate the instrument at a lamp modulation rate of 1 hz or less to achieve the best signal-to-noise ratio.
In a typical prior art design, the AC signal received from the pyroelectric detector is full-wave rectified and then averaged to produce a continuous output. In a circuit of this kind, the averaging time constant must be 5-10 times the modulation period in order to reduce the ripple in the rectified AC signal to acceptable levels. For this reason, prior art designs typically use a lamp modulation frequency of 8 to 10 hz in order to produce an output with a time constant of about 1 second. If these designs were operated at 1 hz (as desired to maximize the lamp emission efficiency, as noted above), the instrument would require an unacceptably long time constant of around 10 seconds. Consequently, such instruments effectively strike a compromise between signal-to-noise ratio and instrument response time.
As mentioned, the pyroelectric sensor typically employed comprises a piezoelectric crystal providing an output voltage responsive to the rate of change of its temperature. Any environmental temperature change communicated to the sensor affects the accuracy of its response to the modulated infrared source. Environmental temperature fluctuations that reach the miniature lamps typically employed also cause changes in the radiation levels emitted. Various prior art designs typically put the lamp directly into the gas stream in the sample tube, and some even put the pyroelectric sensor in the gas stream. To minimize the effect of environmental temperature fluctuations, it would be preferable to thermally isolate both the lamp and the detector from the gas stream, and to provide precise temperature control of both.
Most prior art NDIR instruments, particularly low-cost designs, use a switched unipolar supply for the lamp. This results in accelerated filament degradation known as "DC notching", caused by surface migration of tungsten, and generally severe distortion of the filament shape over time. Both the spectral output and the distribution of the light can be substantially affected, leading to variations in the amount of 4.3 micron radiation reaching the detector. As is well known in the lamp art, operating the lamp on a bipolar supply nearly eliminates the effects of tungsten migration and preserves the original shape of the filament for a much longer time.
It will be appreciated from the foregoing that present-day NDIR instruments can be improved in numerous respects.